Issue 16 Understanding Science

Drug interactions 101

🕒 7 min

Since I started working as a community pharmacist, it has come to my attention that a big part of the general population takes six or more medicines every day, especially the elderly. Although they prepared us for this at university, it still surprised me once I witnessed it in everyday practice. The major problem with polytherapy are drug interactions, which are often neglected, especially in Croatia. The idea of rational pharmacotherapy is just that – to rationalize drug use and consequently assure safer treatments (fewer side effects, minimal risk of sub-dosing or overdosing), less cost to the healthcare system and greater adherence to the therapy; the latter possibly being the most crucial.

Types of interactions

When discussing drug interactions, there are three main types that need to be considered: pharmacokinetic or pharmacodynamic interactions and pharmaceutical incompatibilities. The most common among them are pharmacokinetic interactions, regarding changes in absorption, distribution and elimination of drugs and their metabolites. Pharmacodynamic interactions refer to interactions between two or more active molecules at the place of action – mostly extracellular and intracellular receptors or messenger molecules.

Pharmaceutical incompatibilities are the rarest and include interactions of two or more active molecules given their chemical form – acids, alkali, salt, ions etc. For example, if calcium chloride and sodium carbonate are taken together, the result is precipitation of calcium carbonate and sodium chloride, resulting in the loss of therapeutic effect. Although rare, this interaction is highly dangerous, given that both salts are administered intravenously – calcium chloride for arrhythmias, hypocalcemia and overdosing with both calcium channels blockers and beta blockers (common antihypertensives); while sodium carbonate is used for overdosing on tricyclic antidepressants and for some arrhythmias.

Pharmacodynamic interactions

As previously stated, pharmacodynamic interactions are mostly observed at the enzyme or receptor site. Both receptors and enzymes are polymers, proteins, built of amino acids which have a certain direction in space. Depending on the available side chains of those amino acids, a drug’s active molecules can interact with them and change the shape and function of a given protein. The real beauty lies in the fact that different active molecules can interact with the same receptor or enzyme either at the same or at a different site. Also, active molecules have different binding affinities than the physiological substrates of those enzymes and receptors. The binding affinity of a certain molecule doesn’t only depend on its properties, but also on the environment. One of the most important factors that can change binding affinities is the concentration of the substrate molecule. Even the molecule with the lowest binding affinity for a certain protein will bind to it if its concentration in a given environment is much higher than that of a protein. Similarly, if there are two substrates of the same protein, one with lower binding affinity but much higher concentration, and one with a much higher binding affinity but a much lower concentration, the protein will bind the molecule with higher concentration, regardless of its low binding affinity. This concept enables us to use some molecules as antidotes in case of overdosing or drug poisoning.

Examples from practice

For example, in case of benzodiazepine overdose (the most common anxiolytics used as anxiety, depression and insomnia treatment), administration of flumazenil – a benzodiazepine with a much higher binding affinity – will prevent further binding of other benzodiazepines and prevent overdosing. Benzodiazepines are GABA receptor agonists. GABA receptors bind gamma aminobutyric acid, one of the main inhibitory neurotransmitters of the central nervous system, responsible for mood regulation. One of the most common side effects of benzodiazepines is somnolence (or drowsiness), occurring due to central nervous system inhibition. Inhibition of the central nervous system that is too powerful can lead to cessation of breathing, resulting in death. Although this requires an extremely high concentration of benzodiazepines and occurs just as rarely, the antidote used for such situations is none other than flumazenil – a GABA receptor antagonist.

This pharmacodynamic interaction is one of the positive examples that we used in our favor, but that is not always the case. Pharmacodynamic interactions are more often undesirable than useful, and will usually cause loss of therapeutic effect. One such scenario would be the application of ibuprofen along with acetylsalicylic acid, where ibuprofen would act as a reversible inhibitor for the cyclooxygenase enzyme (COX), required by erythrocytes for the creation of thromboxane, one of the proteins responsible for blood coagulation. Since ibuprofen possesses a greater binding affinity for COX, it will occupy the receptors much faster, preventing the acetylsalicylic acid from binding to the receptors. Regardless, ibuprofen is also rather quick to release from said receptors, making its inhibition of thromboxane synthesis too short-lasting to achieve any significant anticoagulation effect. In the meantime, acetylsalicylic acid, which is an irreversible COX inhibitor, will be eliminated from the erythrocytes and will never reach COX. In other words, simultaneous application of acetylsalicylic acid and ibuprofen will negate the anticoagulant effects of acetylsalicylic acid.

Pharmacokinetic interactions

Pharmacokinetic interactions occur at the level of peripheral active substance absorption, its metabolism, distribution through tissue and ultimately its elimination. It comes as no surprise that these interactions are the most common ones, so let’s look at what we know about them.

The absorption of active substances from the periphery is affected by the very nature of the molecule, the alkalinity of the medium in which we want the absorption to take place, the concentration of the active substance where the absorption takes place, its size and chemical composition and, in some cases, the excipients used to help them along. Substance absorption is commonly affected by the presence of food when medication is applied orally, respiration rate when administered through inhalation, tissue blood flow when applied intravenously and intranasally, as well as skin damage in case of transdermal medication.

The simplest example of interaction between two active substances in which absorption is affected would be the precipitation of levothyroxine and iron within an acidic medium. Levothyroxine is a “replacement” thyroid hormone used to treat hypothyroidism on strict instructions: take at least half an hour before a meal, the same as some iron supplements. Any form of food presence will reduce the absorption of levothyroxine and iron leading to the common practice of taking levothyroxine and iron simultaneously, half an hour before breakfast. This interaction significantly reduces the therapeutic effect of both active substances and has a significant clinical effect.

Interactions in terms of distribution are primarily related to bonding with plasma proteins, which in turn depends on the acidic/alkalic nature of the active substance. Like receptors, two molecules may compete over the same bonding point on plasma proteins leading to one excluding the other. The molecule bonded to a plasma protein as such is “inactive”, meaning its active concentration available for therapeutic effect is increased when another molecule pushes it off a plasma protein. This sort of interaction is crucial in medication of narrow therapeutic widths, such as warfarin – a widely used coagulant with a powerful binding to the plasma molecule. This active substance getting pushed off plasma molecules by another active substance will increase its concentration, which in turn increases the risk of internal hemorrhage, which may end up being fatal.

Drug metabolism is also dependent on the chemical type of molecules and area of application, with interaction most commonly occurring when bonding to the CYP450 enzyme. There are over 2000 CYP450 enzymes present in almost every type of tissue, their activity being most significant in the liver, through which most orally administered medicines must go through. Some active substances act as inhibitors to the CYP3A4 enzyme, which plays a vital role in the metabolism of atorvastatin – a drug used to treat elevated blood fats. When azithromycin inhibits the metabolism of atorvastatin, the concentration of atorvastatin increases along with its therapeutic effect, also increasing the chances of causing side effects tied to it. One of the more dangerous side effects in this scenario would be rhabdomyolysis – a serious disease which affects muscles and reduces their mass.

Is there a happy ending?

Interactions occurring in medication are expected and often predictable. The important thing in terms of preventing and solving side effects is the assessment of their clinical significance, as well as the assessment of individual risks. If we want a safer administration of medication, it is important to secure rational pharmacotherapy for the public, which calls for a tight cooperation of all healthcare experts – and their patients. With that in mind, I would like to take this opportunity and bring this disappointingly common neglect of side effects and drug interactions to your attention, as the only way we can progress in rationalization of pharmacotherapy is – together.

Did you learn something new? Let us know in the comments if you’ve gained a new appreciation for polytherapy, or if you have any questions or examples of your own.

By Đesika Kolarić

Đesika is a pharmacist with an exceptional love for science. Apart from clinical pharmacy, her biggest love is computational biology, which she's currently pursuing through a predoctoral training at Medical university Graz. She loves long walks accompanied by her dog and a good beer.

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